The elucidation of the mechanism of energy transduction in biological systems requires the characterization of the structural, magnetic and spectroscopic properties of (i) dimeric and larger aggregates of chromophores and (ii) molecules in which multiple metal centers lie in close proximity. The purpose of the proposed research is to probe these properties of a series of representative dimeric and binuclear model systems using resonance Raman (RR) spectroscopy. The model systems will include iron, mercury and ruthenium porphyrin dimers and nonporphyrin binuclear complexes containing exchange-coupled iron, copper and chromium ions. The RR spectra of the metalloporphyrin dimers will be obtained at a variety of temperatures and at exctation wavelengths ranging from the Soret to the visible bands. RR excitation profiles will be constructed, and the dispersion of the depolarization ratios of the Raman bands will be measured. The temperature dependence of the linewidths and intensities of the Raman bands will also be measured. The specific aims of these RR studies are threefold: (i) gain new insight into the structural and electronic properties of the dimers; (ii) search for evidence of coupling (exciton interactions) between the constituent porphyrin monomers and (iii) determine whether the presence of low-lying electronic states, such as those which arise as a result of exchange coupling in the iron porphyrin dimers, affects the relaxation of porphyrin vibrational and electronic excited states. Magnetic susceptibility and EPR studies will also be employed in conjunction with the RR studies of several nitrogen-bridged iron porphyrins to probe the exchange interactions in these complexes. The studies of nonporphyrin binuclear transition-metal complexes will involve obtaining RR spectra in the near UV region of the spectrum, where ligand-metal charge-transfer transitions occur. The goal of these studies is to determine whether transitions between the low-lying electronic states which compromise the spin manifold of the exchange-coupled system can be directly observed with RR spectroscopy, as has been tentatively suggested. Taken together, the proposed studies fo the dimeric and binuclear systems should help to provide a more comprehensive picture of the nature of molecular and electronic interactions.